I am really very very late, but I very much enjoyed this book, and Hamlet thanks a lot for selecting it, it is a great read. Beautifuly written, really beutiful prose, though unfortunately I cannot read Japanese so I cannot be sure how much the transaltor contributed to making it so fluent.
I completely agree with you, it is a very romantic story, with the fallen girl turning her back to the world and the love stricken man dying of his love. And indeed it si not easy to understand where the love is springing from, at least in the case of Satoko: what did she find in Kyoiaki beyond beauty? Just the capricious broodyness of a morose teenager? That I found difficult to understand. Similarly, I would have expected some analysis of the resentment if not anger that Satoko must have felt being handed over to the women that would take her child away by Kiyoaki himself. And why does not Kiyo look for Tadeshina to find answers? After Satoko's departure, he is completely self absorbed and almost takes pleasure in self pity - he thinks of disasters and wars that could come and shatetr the situation, but only very late in the day resolves to do something about it.
But on the other hand, this all consuming passion needs no explanation - and there is a sense in which I think the friendship between Honda and Kiyo is also something that deepends out of need of the weakest part - Honda is there to care for the only apparently stronger, luckier boy. And indeed these parallel return, e.g. the picture of "the monster" and Kiyo lying on the grass, the physical suffering of the one mirroring the inner turmoil of the other.
I also very much enjoyed the "background" - the relationship between the Ayakuras and the Matsugaes, the old and the new, but also the relationship between hte masters and their servants, some of which are essential for the actual survival of the family. I only just finished the book, so need to sleep on it, but really cannot wait to read the other three.